flow Add GitHub Copilot instructions file

Configures repository-specific instructions for GitHub Copilot coding agent per best practices.

Changes

Added .github/copilot-instructions.md with repository context:
- Multi-module Maven project structure (flow-server, flow-client, flow-data, etc.)
- Build/test/lint commands for Java 21+ with Jakarta EE 11
- Core architecture: StateNode/StateTree state management, JacksonCodec serialization bridge, Element DOM abstraction
- Key patterns: always pass executeJs parameters via $0/$1 (never concatenate), test-first development, run spotless before committing
- Frontend tooling: Vite dev mode, TypeScript/React/Lit support
- Task guidance: suitable for incremental work, extra caution on core state management changes

The instructions mirror the existing CLAUDE.md structure, adapted for GitHub Copilot's coding agent workflow.

Original prompt

This section details on the original issue you should resolve

<issue_title>✨Set up Copilot instructions</issue_title> <issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository

<Onboard this repo></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes vaadin/flow#22632

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.